The role of magnesium in regulating CCK-8-evoked secretory responses in the exocrine rat pancreas

Abstract

This study investigates the effect of magnesium (Mg2+) on the secretory responses and the mobilization of calcium (Ca2+) and Mg2+ evoked by cholecystokinin-octapeptide (CCK-8) in the exocrine rat pancreas. In the isolated intact perfused pancreas CCK-8 (10(-10) M) produced marked increases in juice flow and total protein output in zero and normal (1.1 mM) extracellular Mg2+ [Mg2+]o compared to a much reduced secretory response in elevated (5 mM and 10 mM) [Mg2+]o. Similar effects of perturbation of [Mg2+]o on amylase secretion and 45 Ca2+ uptake (influx) were obtained in isolated pancreatic segments. In pancreatic acinar cells loaded with the fluorescent bioprobe fura-2 acetomethylester (AM), CCK-8 evoked marked increases in cytosolic free Ca2+ concentration [Ca2+]i in zero and normal [Mg2+]o compared to a much reduced response in elevated [Mg2+]o. Pretreatment of acinar cells with either dibutyryl cyclic AMP (DB2 cAMP) or forskolin had no effect on the CCK-8 induced changes in [Ca2+]i. In magfura-2-loaded acinar cells CCK-8 (10(-8) M) stimulated an initial transient rise in intracellular free Mg2+ concentration [Mg2+]i followed by a more prolonged and sustained decrease. This response was abolished when sodium (Na+) was replaced with N-methyl-D-glucamine (NMDG). Incubation of acinar cells with 10 mM Mg2+ resulted in an elevation in [Mg2+]i. Upon stimulation with CCK-8, [Mg2+]i decreased only slightly compared with the response obtained in normal [Mg2+]o. CCK-8 caused a net efflux of Mg2+ in pancreatic segments; this effect was abolished when extracellular sodium [Na+]o was replaced with either NMDG or choline. The results indicate that Mg2+ can regulate CCK-8-evoked secretory responses in the exocrine pancreas possibly via Ca2+ mobilization. Moreover, the movement of Mg2+ in pancreatic acinar cells is dependent upon extracellular Na+.

Intracellular magnesium transport and regulation in epithelial secretory cells

“…In these experiments, the individual enzyme and total protein secretion were determined by on-line automated methods (152,(154)(155)(156)) based on either fluorimetric or colorimetric measurements. The same effect of high [Mg 2+ ] o is observed in isolated intact pancreas with respect to the pancreatic flow rate and total protein output (44).…”

“…Loading times are short and no detectable compartmentalization is evident. From our studies using dual wavelength fluorimetry with Magfura-2, it would appear that resting [Mg 2+ ] i in both rat and mouse pancreatic acinar cells is about 0.8 mM (41)(42)(43)(44)(45)(46), which is very similar to the value found by other authors with the same technique in pancreatic beta cells (35). Accordingly, using this approach, [Mg 2+ ] i between 0.9 and 1 mM have been reported in heart tissue as measured with Mg 2+ -selective microelectrodes (47)(48)(49) as well as in skeletal muscle as determined by 31 P NMRs (50).…”

“…Concerning the precise mechanism for this action of catecholamines on Mg 2+ movements, an increase in the affinity of the Mg 2+ transporter for intracellular Mg 2+ through cAMPactivated phosphorylation has been postulated (86). (43,44,102,152,153). In these experiments, the individual enzyme and total protein secretion were determined by on-line automated methods (152,(154)(155)(156)) based on either fluorimetric or colorimetric measurements.…”

“…IP 3 , in turn, mobilizes Ca 2+ from endoplasmic reticulum (ER) stores (157), the Ca 2+ -signaling events finally leading to exocytosis and enzyme secretion. Maximal agonist concentrations induce a large and dual Ca 2+ response (40,41,44,152) in which the initial increase has been attributed to Ca 2+ released from intracellular stores and the later sustained phase due to influx of Ca 2+ across the basolateral membrane (158). Submaximal agonist concentrations evoke oscillating cytoplasmic Ca 2+ spikes (46,102).…”

“…Submaximal agonist concentrations evoke oscillating cytoplasmic Ca 2+ spikes (46,102). According to the "two pool" calcium model (159,160) ] i are significantly reduced (both the peak and the plateau phases), together with an inhibition of stimulated 45 Ca 2+ influx (152,43,44). Accordingly, a significant attenuation in both oscillatory spike amplitude and frequency is found after submaximal agonist stimulation (102).…”